While I'm on the subject of the Three Stolen Princesses, I've been reading Jack Zipes' Victorian Fairy Tales. In it is a version of Cinderella by Anne Isabella Ritchie that not only tells the usual story, but also adds how the stepmother managed to hook up with Cinderella's father in the first place.
Which made me realize that the stepmother's story would, as the term is usually used, be considered a Cinderella story. Here you have someone as down as you can be - a woman used to the good life, who's lost it all. No husband, no prospects, two daughters to care for. Suddenly, Mr. Rich comes in and marries her! A real rags-to-riches fairy tale.
Except, of course, that it isn't. Any benefit she might have gotten from her fairy tale ending vanishes due to her harshness and self-interest, without any real need for acting so. After all, she does have a husband who loves her enough to apparently forget about his own daughter, so why should Cinderella be seen as a threat? In contrast, Cinderella has every reason to despise her stepmother and stepsisters, and yet refuses to do so. She won't even stoop to dressing her stepsisters badly for the ball they've forbidden her to attend. Or, as Perrault puts it: "Anyone but Cinderella would have dressed their heads awry, but she was very good, and dressed them perfectly well." (trans. Andrew Lang; see http://www.surlalunefairytales.com/cinderella/index.html) The big difference between Cinderella and her stepmother is not their circumstances and their rise to happiness - which in many ways aren't all that different - but how they see the world, and treat others in it.
In this light, a Cinderella story really shouldn't be considered one where something unexpectedly good happens to someone. It should be one where things happen to someone who is unexpectedly good.